How many days can lice survive off the head? - briefly
Head lice typically survive 24–48 hours without a human host; under optimal temperature and humidity they may persist up to about three days, after which mortality sharply increases.
How many days can lice survive off the head? - in detail
Lice can remain alive without a human host for a limited period that depends on temperature, humidity, and species.
Under normal indoor conditions (20‑25 °C, 40‑60 % relative humidity) adult head lice survive approximately 24‑48 hours. They become inactive after about 12 hours of starvation and die within two days if they cannot locate a scalp.
Cooler, more humid environments extend survival. At 10‑15 °C with humidity above 80 %, adults may persist for up to five days, and in laboratory settings they have been recorded alive for up to seven days. Extreme cold (below 0 °C) or very low humidity (under 30 %) shortens viability to a few hours.
Nymphs are less tolerant than adults. They typically die within 12‑24 hours under standard indoor conditions and rarely exceed 48 hours even in optimal humidity.
Eggs (nits) are more resistant. An unhatched egg can remain viable for several weeks if kept in a moist environment, but it requires a suitable temperature range (15‑30 °C) to complete development. Once detached from a hair shaft, the egg loses the protective coating and usually fails to hatch within a few days.
Key points for practical assessment:
- Typical indoor setting: 1‑2 days for adults, <1 day for nymphs.
- Cool, humid environment: up to 5‑7 days for adults, slightly longer for nymphs.
- Eggs: viable for several weeks in moist conditions, but unlikely to hatch after removal from hair.
Therefore, the maximum period a head louse can survive away from a scalp does not exceed one week, with most circumstances limiting survival to less than two days.